1. Field of the Invention
This invention relates to materials and processes for deposition of thin films on solid substrates. In particular, this invention relates to materials and processes for deposition of tungsten-containing thin films on solid substrates. This invention also relates to methods and materials for making electrically conducting, conformally deposited films for fabrication of devices in the areas of microelectronics.
2. Description of the Related Art
Tungsten nitride, WNx, is considered to be a good barrier against diffusion of copper in microelectronic circuits. WNx can also be used in electrodes for thin-film capacitors and field-effect transistors. WNx has been made by reactive sputtering, but the uniformity of film thickness inside narrow features (“step coverage”) is not expected to be adequate for use in future microelectronic devices having narrow features with high aspect ratios.
Atomic layer deposition (also known as atomic layer epitaxy) is a process for depositing thin layers of solid materials from two vapor precursors. The surface of a substrate onto which a film is to be deposited is exposed to a dose of vapor from one precursor. Then any excess unreacted vapor from that precursor is pumped away. Next, a vapor dose of the second precursor is brought to the surface and allowed to react. This cycle of steps can be repeated to build up thicker films. Typically, each precursor contributes a portion of the atoms to the deposited film. One particularly important aspect of this process is that the ALD reactions are self-limiting, in that only a certain maximum thickness can form in each cycle, after which no further deposition occurs during that cycle, even if excess reactant is available. Because of this self-limiting character, ALD reactions produce coatings with highly uniform thicknesses. Uniformity of ALD film thickness extends not only over flat substrate surfaces, but also into very narrow holes and trenches. This ability of ALD to make conformal films is called “excellent step coverage.”
Coatings of WNx made by ALD from WF6 and NH3 have good step coverage. A disadvantage of this process is that WF6 and/or its reaction byproduct, HF, can attack substrates made of Si or SiO2. This reaction can also generate unwanted particles of ammonium fluoride byproduct that may cause defects in a semiconductor product. Also, this process can leave the WNx surface with a fluorine residue that may impede adhesion of copper to the surface. In particular, adhesion of Cu deposited by CVD is often considered to be poor in part because of fluorine contamination at the interface between the tungsten nitride and the copper. Loss of adhesion can cause severe loss of yield in manufacturing or reliability problems during operation of a semiconductor device.
Molybdenum nitride layers may be used along with alternating layers of silicon to make mirrors for X-rays. ALD would be an ideal method for depositing the MoNx and silicon layers with the required highly uniform thicknesses needed in an X-ray mirror.